1. Field of the Invention
This invention relates to solid oxide fuel cells, in particular, interconnectors comprising a superalloy metallic layer and a metal layer, which metal layer protects the superalloy metallic layer against increases in electrical resistivity in the fuel environment of the anode facing face of the interconnector due to the formation of chromia (Cr.sub.2 O.sub.3). In particular, the metal layer comprises a metal which reacts with chromia to form an oxide phase on the superalloy metallic layer.
2. Description of Prior Art
It is well known that nickel is perfectly satisfactory as a constituent of an anode electrode in solid oxide fuel cells in which hydrogen, as well as reformed methane, is used as a fuel. It can readily be shown that, on the one hand, in the fuel atmosphere at the anode electrode, neither nickel oxide nor nickel carbide can form. On the other hand, most superalloys, that is alloys which are resistant to oxidation at high temperatures, such as austenitic stainless steel and INCONEL.TM., contain a significant amount of chromium. The partial pressure of oxygen at the anode electrode is usually high enough to form chromium oxide or chromia (Cr.sub.2 O.sub.3). Although chromium oxide (chromia) scale does not grow rapidly, its resistance is rather high and, thus, it is desirable that its thickness be as small as possible. In our work on fuel cell stack testing, we have observed that the oxide coating is usually thicker on the fuel side (anode side) than on the air side (cathode side) of the interconnector between cell units. We believe that this may be due to the fact that water formed by the electrochemical reaction of fuel and oxygen adversely affects the kinetics of oxide growth. We have, in fact, observed in our work that the resistance of the interconnector on the fuel side is actually greater than the resistance on the air side, both immediately after testing and at room temperature. The solid oxide fuel cell interconnectors of this invention address this issue by preventing an increase in the net interconnector resistance through the formation of an electronically conducting oxide phase on the fuel side of the interconnector.
Solid oxide fuel cells, like other fuel cells, comprise an anode electrode, a cathode electrode, and an electrolyte disposed between the anode electrode and the cathode electrode. In contrast to other types of fuel cells, for example, molten carbonate fuel cells, solid oxide fuel cells operate at relatively high temperatures, typically greater than about 800.degree. C. Accordingly, the interconnector materials must be able to withstand such temperatures.
One solution to the problem of metallic interconnector oxidation in solid oxide fuel cells is taught, for example, by U.S. Pat. No. 4,950,562 which teaches a solid electrolyte type of fuel cell having an interconnector comprising a heat resistant alloy substrate coated on its surface with a composite metal oxide of the perovskite-type structure, that is La.sub.1-x M.sup.1.sub.x M.sup.2 O.sub.3 wherein M.sup.1 is an alkaline earth metal, M.sup.2 is Co, Fe, Mn, Ni, or Cr and x is greater than or equal to zero and less than 1. U.S. Pat. No. 5,411,767 teaches a method for producing interconnectors for electrically connecting unit cells of a solid electrolyte type fuel cell in which the interconnector material, a perovskite-complexed oxide, is thermally sprayed onto the surface of an electrode of a solid electrolyte type fuel cell by plasma thermal spraying. An interconnector made of lanthanum chromite or lanthanum oxide and chromium oxide doped with copper, zinc, calcium or strontium for a solid oxide fuel cell is taught by U.S. Pat. No. 5,480,739. See also U.S. Pat. No. 4,874,678 and U.S. Pat. No. 4,888,254, both of which teach interconnects of lanthanum chromite doped with calcium, strontium, or magnesium for use in connection with solid oxide electrolyte fuel cell stacks; U.S. Pat. No. 5,034,288 which teaches a solid electrolyte fuel cell stack comprising a metallic bipolar plate comprising a nickel-based alloy and coated on the oxygen side with a lanthanum/manganese perovskite applied by plasma spraying; U.S. Pat. No. 4,997,727 which teaches an interconnect for a solid electrolyte fuel cell stack constructed of INCONEL.TM. X; and U.S. Pat. No. 5,496,655 which teaches a bipolar interconnector manufactured from NiAl or Ni.sup.3 Al coated with strontium or calcium-doped lanthanum chromite.
In contrast thereto, the interconnects for solid oxide fuel cells in accordance with this invention are substantially lower in cost while providing high conductivity relative to other known interconnects.